Hydrogen is widely used in many industrial fields including petroleum refining and chemical engineering, but especially in recent years it has become noted as a future energy source, and research is being conducted with particular emphasis on fuel cells. However, hydrogen gas has high volume per quantity of heat, as well as high energy requirement for liquefaction, which poses the problem that its storage and transport in that form have been difficult (see Non-patent document 1, for example). Techniques are therefore being sought for efficient transport and storage of hydrogen, when fuel cells are to be used in vehicles such as fuel cell automobiles or as distributed power sources.
One method that has been proposed is to use hydrogen in the form of liquid hydrogen for storage and transport, but since the liquefaction temperature is a cryogenic temperature of −253° C. it is difficult to handle, and massive energy is required for liquefaction, thus lowering the overall energy efficiency (see Non-patent document 2, for example).
Methods of using hydrogen as a high-pressure gas for transport are also being implemented. Such methods, however, entail problems such as requiring the handling of dangerous high-pressure gas, with a very large the volume even at the extremely high pressure of 35 MPa, making it difficult to achieve small sizes (see Non-patent document 3, for example).
One powerfully effective method is storage in hydrogen storage alloys. However, the hydrogen storage capacity of hydrogen storage alloys is usually about 3%, which is not only insufficient for use in vehicles and the like, but also results in excessively increased weight. Hydrogen storage alloys also have reduced energy efficiency because of the need for large amounts of heat during hydrogen desorption, while the system also becomes complicated (see Non-patent document 4, for example).
The use of hydrogen storage materials is also considered as a means of transporting hydrogen gas in a compact manner (see Patent document 1). Because this method allows hydrogen desorption to be accomplished at ordinary temperature and thus simplifies the system, while it generally does not require heat for hydrogen desorption and thus has high energy efficiency, the development of such materials is being actively pursued. It has also been reported that materials such as carbon nanotubes and carbon nanofibers exhibit high storage capacity (see Non-patent document 5, for example). However, their reproducibility is in question, and at the current time it cannot be said that the development of hydrogen storage materials with sufficient reproducibility and high storage capacity has been achieved.
A demand therefore exists for development of materials with high storage capacity, and hence materials with fine pore sizes of the same level as hydrogen are being studied as materials with high storage capacity. Examples of such materials include the aforementioned carbon nanotubes and carbon nanofibers, but also other primarily carbon-based materials. Materials other than carbon have also been reported, including boron nitride nanotubes (Non-patent document 6) and porous complexes (Non-patent document 7). However, despite reports of some materials exhibiting high storage capacity, the currently available data is not sufficiently reliable.